Someone committed many sins in his life, and this was confirmed by the bad end of his life, the thing that means that he will be in Hellfire. How can he be rewarded by giving charity on his behalf and supplicating Allaah for him? How will this change his destiny while he met a bad end as I mentioned earlier? May Allaah The Almighty Reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Actually, deciding that someone will be in Hellfire cannot be known in any way except with a text from the Noble Quran or the honorable authentic Sunnah. That is especially because this is an unseen matter which no one knows except Allaah The Almighty Who Knows the destiny of everyone. So, if Allaah The Almighty Wills, He Will Forgive him, and if He Wills, He Will Punish him. It was narrated that Ibn ‘Abbaas said: “No one should decide what Allaah The Almighty Would Do with His creation nor decide that they will enter Paradise or Hell."

Regardless of the sins of the disobedient person, Allaah The Almighty May Forgive him if He Wills as an honor and favor from Him. Also, He May Punish him out of His justice. In fact, no one knows what this person has really done except Allaah The Almighty. Allaah The Almighty Says (what means): {Indeed, Allaah Does not Forgive association with Him, but He Forgives what is less than that for whom He Wills.} [Quran 4:4]

Furthermore, you have to know that the dead person has reached the result of his deeds and you must not speak ill of him in any way for the Prophet, sallallaahu ‘alayhi wa sallam, said: “Do not abuse the dead, because they have reached the result of what they offered (of deeds)." [Al-Bukhari[

Giving charity on the behalf of the one who died in the state of faith or supplicating for him is permissible and Allaah Willing he will be rewarded for such good acts. Actually, it is worthier of his children do that on his behalf.

It was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “When a man dies, all his good deeds come to an end except three: an ongoing charity; beneficial knowledge; and a righteous child who will pray for him.”  [Muslim]

In another narration, it was narrated that a man asked the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, saying: “‘My mother died suddenly and she did not leave a will, but I think that if she could have spoken, she would have given in charity. Will she have a reward if I give in charity on her behalf?’ The Prophet, sallallaahu ‘alayhi wa sallam, said, ‘Yes. Give in charity on her behalf.’” [Al-Bukhari and Muslim]

Allaah Knows best.